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Position of round slot

I am trying to grasp the concept of the round slot position callout. As per ASME Y14.5M 94 GD&T standard (I don't have newer one), section 5.10.1 - "Noncircular features at MMC", point "c" states: "In terms of the boundary for an elongated feature. While maintaining the specified size limits of the elongated feature, no element of its surface shall be inside a theoretical boundary of identical shape located at true position. The size of boundary is equal to the MMC size of the elongated feature minus its positional tolerance." I have to check part against the drawing where hole is datum B and length of the round slot ( slot is in the same plane as the hole) is specified as datum C (shouldn't the width be the datum not the length?). Minimum size of the slot is 7.00 X 11.00. Both, width and length have positional callouts: length is 0.25 MMC, A, B MMC (A is a plane in which both features are) and width is 0.25 MMC, A, B MMC, C MMC. As per ASME, boundaries for this slot are 6.75 X 10.75 so in case of the slot at minimum size the maximum allowable shift in each direction is 0.125. My question is: what happens when the size of the slot increases? If, for example, my slot is now 7.02 X 11.02 does that mean that allowable shift increased by 0.02 or that the boundary decreased by 0.02 ( which means allowable shift increased only by 0.01)? Also, is MMC modifiers applied to datums B and C correct or should they both be RFS?

I think I figured out first part, if the size increases by 0.02 than allowable shift increases by half of it so 0.01. I am still not sure if MMC modifiers associated with B (hole) and C (length of the slot) are correct? If they are than any bonus from them increases allowable shift by only half of it.

The boundary concept simply specifies a continuing MMC size collective of the height and width. That size boundary can shift in location as specified by the specified position tolerance. Therefore, the inner boundary for an inner FOS is equal to MMC size, both width and height minus position tolerance with respect to the specified datums.

It is often best to view the inspection requirements in terms of a functional check gage. see figure 5-47

"My question is: what happens when the size of the slot increases?" For an internal feature of size the surfaces move away from the internal boundary (size and location tolerance). As long as the slot FOS does not exceed LMC specified sizes or shift in location out of the specified position tolerance then it is an acceptable feature. If there is an MMC modifier the effects of increased position tolerance should be accounted for.

This concept simply allows any FOS to be treated like a hole or shaft feature.

Be aware that the most recent ASME standards do away with the requirement to define "BOUNDARY" with the specification.

Tell me and I forget. Teach me and I remember. Involve me and I learn.

Thank you Kelly. I still would like to know if specifying length instead of width of the round slot as a datum is correct? I simply look at this from measuring perspective where, in order to rotate to slot, 2 flat sides of the slot are measured to create mid-plane for the alignment rotation. If I would to rotate to length instead of width than I have to measure 2 radius's of the slot and rotate to the line created from their centers which is less accurate than measuring 2 planes on the flat sides.

I still would like to know if specifying length instead of width of the round slot as a datum is correct?

I don't know the design intent. Most correctly, datum features should be specified based on function in terms of fit and function within the target assembly. Datums are those features which do the alignment, orientation and positioning of one mating part to another.

What is easiest to measure is meaningless. If the long radius to radius feature of the oblong hole aligns, positions the part in the target assembly then that is the best datum for alignment.

Tell me and I forget. Teach me and I remember. Involve me and I learn.